Conventionally, as methods of this type, there are known a maximum value projecting and displaying method, an integrated value projecting and displaying method, etc., as shown in the following papers (1) and (2).    (1) P. J. Keller, B. P. Drayer, E. K. Fram, K. D. Williams, C. L. Dumoulin, S. P. Souza “MR Angiography with Two-dimensional Acquisition and Three-dimensional Display”; Radiology, November 1989, pp. 527–532    (2) Jun Nagai and Hideo Ando “Three-dimensional CT Angiography”; May 30, 1994,     p. 38, published by Medical Science International Co., Ltd.
The maximum value projecting and displaying method, however, had a problem that the method was inferior in terms of separative display of internal organs because nothing but a projection image based on the maximum value of density was displayed. On the other hand, as shown in FIG. 21, the integrated value projecting and displaying method had a problem that the shadow of an internal organ d2 which is small in the distance of passage of projection line L was buried under the shadow of an internal organ d1 which is large in the distance of passage of projection line L because the shadow of the small internal organ d2 was thinner than the shadow of the large internal organ d1, etc. It was difficult to display those organs d1 and d2 under the condition that they are discriminated from each other. In FIG. 21, e designates a view point; L, view lines (projection lines); P, a plane of projection; and 11 to 14, sectional images. Relational formulae among the view point e, the view line L and the projection plane P are described in JP-A-8-16813.
An object of the present invention is to provide a projection image forming and displaying method and apparatus which is excellent in separative display of portion of object to be examined such as internal organs, etc. in sectional images, so that projection images of the respective parts sharply separated are obtained.